An example of a rolling bearing unit used for an automobile or the like and heretofore known is shown in FIGS. 3 and 5, for example, as in U.S. Pat. No. 5,011,233. Referring to FIG. 3, reference numeral 30 designates a first inner ring member, 36 a second inner ring member, 38 a nut for pressing and fixing the second inner ring member 36, and 40 an outer ring member. The first inner ring member 30 has, so that a seal 45 secured to an inner end 40a of the outer ring member 40 may be slidably moved, a peripheral shoulder 34 formed in an axial outer surface 33b of a root portion 33a of a radial flange 33 for fixing a wheel provided in an outer peripheral surface of an inner end 31 thereof, and a track surface 35 formed in a peripheral surface of a shaft portion 30a smaller in diameter than that of the peripheral shoulder 34. The second inner ring member 36 is fitted in a step portion 48 formed in the end of the shaft portion 30a of the first inner ring member 30 and has a track surface 37 formed in an outer surface. The outer ring member 40 has a radial flange 39 for fixing a vehicle provided in an outer peripheral surface thereof and track surfaces 42 and 41 provided opposite to and in inner peripheral surfaces of both the track surfaces 35 and 37 of the first and second inner ring members 30 and 36, respectively. Rolling members 43 and 44 are respectively arranged between the track surfaces 35 and 42 provided opposite to each other in the first inner ring member 30 and the outer ring member 40, respectively, and between the track surfaces 37 and 41 provided opposite to each other in the second inner ring member 36 and the outer ring member 40. Further, a lip 46 of a seal 45 secured to an inner end 40a of the outer ring member 40 to prevent a leakage of a lubricant such as grease, and prevent an entry of foreign matter such as dust and water into the bearing comprises a main lip 46a directed externally of the bearing and sliding on the peripheral shoulder 34, an auxiliary lip 46b directed internally of the bearing and sliding on the peripheral shoulder 34, and a side lip 46c sliding on the axial outer surface 33b of the radial flange 33 for fixing a wheel. The bearing unit has a cover 47 secured to the outer end 40b of the outer ring member 40 for covering the shaft end, and a retainer 48a for retaining the rolling members 43 and 44. As will be apparent from the drawing, in the rolling bearing unit, a diameter D of the shaft portion 30a of the first inner ring member 30 is formed to have a smaller diameter than a bottom diameter D1 of the track surface 35. This is because of consideration that in assembly of the rolling bearing unit of this kind, when the sub-assembly including the outer ring member 40, the rolling members 43 and 44, the retainer 48a and the seal 45 is incorporated into the rolling bearing unit, the rolling members 43 and 44 are moved to the peripheral shoulder 34 and the seal 45 is installed in an adequate position.
In the case where the diameter of the shaft portion 30a of the first inner ring member 30 is formed to be smaller than the bottom diameter D1 of the track surface 35 in consideration of the assembling property of the aforementioned rolling members, the rolling member 44 causes a diametral clearance between the shaft portion 30a and the outer ring member 40 to be large when the rolling member is not run on the track surface 35 before the sub-assembly is incorporated into the shaft portion of the first inner ring completely, as will be understood from FIG. 5. Therefore, when the sub-assembly is further incorporated into the shaft portion so that the rolling members 43 and 44 are run on and installed on the track surfaces 37, 35, respectively the outer ring member sometimes greatly deviates in a diametral direction. The main lip 46a of the seal 45 is formed externally of the bearing as described above in consideration of preventing an entry of grease, contaminated water and the like from the outside of the bearing. As a result, when the main lip 46a of the seal 45 is incorporated as the sub-assembly article into the rolling bearing unit 30a during assembly, it is sometimes caught in a corner 34a (FIG. 3) of the peripheral shoulder 34 of the first inner ring member 30 so that the main lip 46a is bent backward in the direction opposite to the rolling member and installed in the peripheral shoulder 34, posing a problem in that sealing properties between the first inner ring member 30 and the outer ring member 40 fail to be maintained.
As a means for preventing the phenomenon as described above, a rolling bearing unit as shown in FIG. 4 is proposed. In this rolling bearing unit, a slanting surface 49 serving as a guide portion for the main lip 46a is formed between the peripheral shoulder 34 of the first inner ring member 30 and the track surface 35. When the slanting surface 49 is formed between the peripheral shoulder 34 of the first inner ring member 30 and the track surface 35 as described above, it needs to be designed in a manner of that in the length of the whole rolling bearing unit, a portion of the length required for the slanting surface is extended in an axial direction. As a result, there poses a problem that the weight of the whole rolling unit increases.